Investigations of the possible direct use of powdered coal as a motor fuel have been conducted for many years without commercial success. The principal problems have been abrasion due to the ash content of the coal and incomplete combustion due to the low burning rate of the coal particles.
Extraction of coal by various solvents has been studied even more intensively for several decades, but the possibility of using these coal extracts directly as motor fuel has not resulted in a commercial product.
The advantages of using coal extract directly as a motor fuel are great, however, since there is high cost and much loss of useful energy in conversion of coal into gasoline and diesel fuel.
Possible reasons for past lack of interest in the possibilities of such use are:
1. Low burning rate of product from high severity hydrogen donor solvent processes such as Solvent Refined Coal and Pott-Broche processes;
2. High residual ash content of extract from alkali extraction processes;
3. Agglomeration of colloidal size coal particles in hydrocarbon fuels.
High severity processes remove a majority of the oxygen from the coal, and therefore greatly reduce the burning rate of particles of a given size. In addition, product from the Pott-Broche or Solvent Refined Coal processes is obtained as a molten solid, which forms large lumps as it solidfies. Grinding it to a micron-range particle size that might have suitable burning characteristics is expensive.
Examples of high severity extraction processes are: Pott-Broche, Solvent Refined Coal, and processes extracting with alkali in low molecular weight alcohols (C.sub.1 -C.sub.4) at temperatures of 300.degree. C. or above.
To be suitable for use in a motor fuel, four factors are important with respect to the use of coal;
1. Ash content
2. Combustion characteristics
3. Fuel system tolerances
4. Stability of fuel suspension.
It has already been noted that ash present in coal can be a serious problem in motors due to its abrasive characteristics. The three remaining factors all relate, at least in part to the particle size of the coal. For example, although burning rate is related to chemical composition of the coal, available data indicate that particles smaller than 30 microns from bituminous coal will burn satisfactorily in diesel engines, and particles from sub-bituminous coals or lignite smaller than 30 microns should burn satisfactorily in either diesel or spark ignited engines.
While smaller particle size is desirable from the standpoint of combustion and suspension stability, too small particle size is undesirable from the standpoint of fuel viscosity. This is particularly true of particles smaller in diameter than one micron.
Further, most current gasoline and diesel engine have fuel filters designed specifically to remove suspended particles. A typical commercial diesel engine has a filter with 20 micron pore openings; and therefore will not pass many particles above about 10 microns in size. Filters with slightly larger openings can easily be substituted, but a major increase in filter pore opening is undesirable and may be impossible because the filter would then pass dirt and other abrasive particles picked up by the fuel in storage and shipment.
Accordingly, it is an object of the present invention to provide a liquid motor fuel incorporating refined, particulate carbonaceous material, the carbonaceous material being characterized by forming a relatively stable suspension and having good burning characteristics, without being harmful to the motor.